1. Field of the Invention
The present invention relates generally to the fields of air purification and heating, ventilating, and air conditioning, and more particularly, to systems and methods for filtering or removing biological and chemical contaminants from air, such as an air stream being conditioned for input to an inhabited building or selected rooms in a building or protective shelters, and for deactivating the filtered or captured contaminants.
2. Relevant Background
Maintaining acceptable indoor air quality within commercial and residential buildings is a serious and often difficult challenge facing today's industrial society. Indoor air quality is generally the condition of air in an enclosed space with respect to contaminants or pollutants that have entered the air and that can cause health problems for inhabitants of the enclosed space. Health authorities are concerned with contaminants that are respirable particles, which are typically 10 microns or less is size, and that are often drawn into and distributed by the building ventilation system where people breathe in the contaminants. The challenge facing designers of building ventilation systems in to condition outside air to provide air with an acceptable level of contaminants for the building and to maintain an acceptable level of indoor quality for recirculated air.
To maintain indoor air quality, ventilation systems need to be adapted to control a wide variety of contaminants. In typical applications, the contaminants may include bioaersols including allergens (e.g., pollens, fungi, mold spores, and the like) and pathogens (e.g., bacteria and viruses), respirable particles such as chemical pollutants, and vapors and gases (e.g., volatile organic compounds, radon, and the like). In recent years, the use of chemical and biological agents as weapons in war and by terrorists has given rise to a need for ventilation systems designed to harden the building against such attacks by attempting to prevent introduction of potentially deadly contaminants into a building. Examples of these agents include anthrax spores, nerve agents, mustard gas, phosgene, cyanogen chloride, chlorine, bacteria such as salmonella and E. coli, and viruses such as small pox. Designing a single ventilation system that effectively controls this broad spectrum of potential airborne contaminants has proven to be a very difficult task that has not yet been successfully accomplished by broadly applicable means due to the diversity in the physical and chemical characteristics of the contaminants. Many of these contaminants, such as anthrax and other spore contaminants, are notoriously resistant to deactivation by chemical, radiation, and thermal techniques.
Conventional methods of controlling contaminants include physical filtration (such as with a high efficiency particulate air (HEPA) filter), electrical filtration (such as with an electrostatic precipitator), thermocatalytic oxidation, photocatalytic oxidation, carbon adsorption, or sequential combinations of these techniques. While these methods can provide useful contaminant control, these methods are often only effective against a single contaminant or for select contaminants. Further, these existing methods often are expensive to implement and maintain and typically require significant modification and upgrades of conventional heating and ventilation systems. For example, existing military systems utilize HEPA filters for particulate contaminants in conjunction with carbon filters that handle gaseous contaminants. These multiple filter systems are large, heavy, and costly to produce, install, and maintain in part because the systems demand high fan pressure and frequent replacement. Unfortunately, most conventional heating and ventilation systems are currently not built to handle the high airflow resistance of HEPA filters and require major system modifications, such as installing additional fans, modifying duct work, and, in some cases, installing structural supports for the heavy equipment.
The existing systems fail to provide all of the desired features of a filter system (such as the design criteria presented by the United States Joint Forces Chemical/Biological Defense Command for Collective Protection as detailed in the Collective Protection Master Plan Summary, DOD Chemical & Biological Defense Program AFRL/MLQ, 139 Barnes Drive, Suite 2, MS37, Tyndall AFB, Florida USA 32403). These features include simultaneously controlling gases, aerosols, and particulates including bacteria, viruses, and spores with a single filter. Further, it is desirable that the filter provide continuous agent destruction (i.e., be regenerable) such that the filter's efficiency remains relatively constant (e.g., does not decrease over its service life such as by the filter consuming one or more essential decontaminating agents) and the filter does not require frequent maintenance, manual cleaning, and/or replacement. It is also important to minimize the need for maintenance when the filtered contaminants may be dangerous or hazardous, and in this regard, it is desirable that use of the filter does not result in a filter that has captured numerous contaminants and has become a concentrated disposal hazard.
Hence, there remains a need for an improved filter or filter system for use in building or other ventilation systems to provide protection against biological and/or chemical agents or contaminants, such as the types of contaminants that may be released in a terrorist attack. Further, it is desirable that such an improved filter or filter system meets demands for low cost, reduced size, low maintenance, and reduced energy demands. It is also desirable that the filter system be “dual use” in that it serves a useful air quality function in normal day-to-day operations as well as a protective function in a biological or chemical attack. Specifically, it is desirable that such a filter or filter system be compatible with conventional building heating and ventilation systems while providing filtration rates for contaminants that are comparable to those achieved with many HEPA filters, such as in excess of 99 percent and even in excess of 99.99 percent thereby avoiding the creation of a hazardous waste disposal problem.